Domains in addresses had very lax validation, which allowed them to
include characters like "/".
That could be exploited to trigger a daemon panic: a MAIL FROM
"x@evil/foo" could be accepted and, on failed delivery, a DSN queued.
Attempting to deliver the DSN would cause an MTA-STS cache lookup, which
would panic because the domain includes a "/" and crash the server.
Because the DSN would be queued, the server would crash again on
restarts, and only self-recover after give_up_send_after (20h by
default).
To fix this, we now explicitly validate the domain when normalizing it,
make MTA-STS cache not panic on invalid domains, and also make the SMTP
courier fail permanently on invalid domains.
Thanks to John JEAN <john@wargan.com> for finding and reporting this
problem. The detailed report can be found in
https://johnjean.io/advisories/chasquid-unauthenticated-remote-dos/.
Today, we close the connection after 10 errors. While this is fine for
normal use, it is unnecessarily large.
Lowering it to 3 helps with defense-in-depth for cross-protocol attacks
(e.g. https://alpaca-attack.com/), while still being large enough for
useful troubleshooting and normal operation.
As part of this change, we also remove the AUTH-specific failures limit,
because they're covered by the connection limit.
This patch adds a new integration test, which executes various small
dialogs, to cover corner cases that are not well covered (according to
our coverage report).
For example, "EHLO" without domain, or invalid DATA.
While we could do them via Go tests, this way is more realistic, and the
tests are easier to write.
Alberto Bertogli